将一个list分成多个list

系统管理员 2022-01-14 06:11 1466阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

将一个list或其他集合按需分成多份，例如，分批发送等，是很常见的需求，但是Java传统的集合操作似乎没有提供这一支持。

幸好，google guava 和apache commons collections都提供了相应的实现。

方法一：guava实现，将一个list按三个一组分成N个小的list

代码块 [@Test][Test]  public void givenList\_whenParitioningIntoNSublists\_thenCorrect() \{  List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);  List<List<Integer>> subSets = Lists.partition(intList, 3);

List<Integer> lastPartition = subSets.get(2);  List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);  assertThat(subSets.size(), equalTo(3));  assertThat(lastPartition, equalTo(expectedLastPartition));  \} 方法二：guava分割其他collections

代码块 [@Test][Test]  public void givenCollection\_whenParitioningIntoNSublists\_thenCorrect() \{  Collection<Integer> intCollection = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);

Iterable<List<Integer>> subSets = Iterables.partition(intCollection, 3);

List<Integer> firstPartition = subSets.iterator().next();  List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(1, 2, 3);  assertThat(firstPartition, equalTo(expectedLastPartition));  \} 以上需要注意的是，partition返回的是原list的subview.视图，也即，原list改变后，partition之后的结果也会随着改变。

代码块 [@Test][Test]  public void givenListPartitioned\_whenOriginalListIsModified\_thenPartitionsChangeAsWell() \{  // Given  List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);  List<List<Integer>> subSets = Lists.partition(intList, 3);

// When  intList.add(9);

// Then  List<Integer> lastPartition = subSets.get(2);  List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8, 9);  assertThat(lastPartition, equalTo(expectedLastPartition));  \} 方法三：使用apache commons collection

2.partition后的结果同样是原集合的视图。

方法四：Java8方法。(在Java stream 中有讲。)

a.通过grouping by

代码块 [@Test][Test]  public final void givenList\_whenParitioningIntoNSublistsUsingGroupingBy\_thenCorrect() \{  List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);

Map<Integer, List<Integer>> groups =  intList.stream().collect(Collectors.groupingBy(s -> (s - 1) / 3));  List<List<Integer>> subSets = new ArrayList<List<Integer>>(groups.values());

代码块 Map<Integer, List<Person>> personGroups = Stream.generate(new PersonSupplier()).  limit(100).  collect(Collectors.groupingBy(Person::getAge));  Iterator it = personGroups.entrySet().iterator();  while (it.hasNext()) \{  Map.Entry<Integer, List<Person>> persons = (Map.Entry) it.next();  System.out.println("Age " + persons.getKey() + " = " + persons.getValue().size());  \} b.通过partition by

代码块 @Test  public void givenList\_whenParitioningIntoSublistsUsingPartitionBy\_thenCorrect() \{  List<Integer> intList = Lists.newArrayList(1, 2, 3, 4, 5, 6, 7, 8);

Map<Boolean, List<Integer>> groups =  intList.stream().collect(Collectors.partitioningBy(s -> s > 6));  List<List<Integer>> subSets = new ArrayList<List<Integer>>(groups.values());

List<Integer> lastPartition = subSets.get(1);  List<Integer> expectedLastPartition = Lists.<Integer> newArrayList(7, 8);  assertThat(subSets.size(), equalTo(2));  assertThat(lastPartition, equalTo(expectedLastPartition));  \} 按照成年人与未成年人分组：

代码块 Map<Boolean, List<Person>> children = Stream.generate(new PersonSupplier()).  limit(100).  collect(Collectors.partitioningBy(p -> p.getAge() < 18));  System.out.println("Children number: " + children.get(true).size());  System.out.println("Adult number: " + children.get(false).size()); 注意：

1.Java8方式，分组后的list不再是原list的视图。所以，原list的改变不会影响分组后的结果。

2partitioningBy 其实是一种特殊的 groupingBy，它依照条件测试的是否两种结果来构造返回的数据结构，get(true) 和 get(false) 能即为全部的元素对象

转载于:https://my.oschina.net/u/1000241/blog/3020830

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220114/e3386df0f77c4bb5937323e34257920f.png
[Test]: https://my.oschina.net/azibug